Arthrogenic muscle inhibition manifests in thigh musculature motor unit characteristics after anterior cruciate ligament injury

ABSTRACT

Joint trauma induces a presynaptic reflex inhibition termed arthrogenic muscle inhibition (AMI) that prevents complete activation of muscles. Reduced motor unit (MU) output is a hypothesised mechanism for persistent strength deficits. The objective of this study was to determine MU characteristics of thigh musculature and determine how they change with anterior cruciate ligament (ACL) injury compared to healthy controls. A randomised protocol of knee flexion/extension isometric contractions (10–50% maximal voluntary isometric contraction) was performed for each leg with surface EMG 5-pin array electrodes placed on the vastus medialis, vastus lateralis, semitendinosus and biceps femoris. Longitudinal assessments for average rate coding, recruitment thresholds and MU action potentials were acquired at 6-month intervals. With exception of the vastus medialis, all thigh musculature of ACL-injured demonstrated smaller MU action potential peak-to-peak amplitude. For average rate coding, ACL-injured demonstrated lower coding rates than Controls for the quadriceps (p < .05) and higher rates than Controls for the hamstrings (p < .05). These MU characteristics were different from Controls after ACL reconstruction up to 12 months post-surgery, yet maximal strength increased during this time frame. As thigh MU characteristics are known across phases of ACL rehabilitation, future studies can assess these patterns of motor control and their potential to determine risk of re-injury. Further, future rehabilitation can target specific intervention programmes to restore motor control.

Highlights

• Motor unit strategies of arthrogenic muscle inhibition are characterised for the first time via decomposed EMG.

• Motor unit deficits of thigh musculature persist throughout all phases of ACL rehabilitation, even after return-to-sport.

• After ACL injury, motor unit sizes at similar recruitment thresholds were smaller than those of healthy controls.

KEYWORDS:

• Rehabilitation
• motor control
• biomechanics
• injury and prevention

Acknowledgements

Its contents are solely the responsibility of the authors and do not necessarily represent the official view of NIH.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by Eunice Kennedy Shriver National Institute of Child Health and Human Development: [Grant Number K12HD065987]; National Center for Advancing Translational Sciences: [Grant Number UL1TR002377]; National Institute of Arthritis and Musculoskeletal and Skin Diseases: [Grant Number L30AR070273, R01AR056259]; Mayo Clinic Graduate School of Biomedical Sciences and the University of South Florida Center for Neuromusculoskeletal Research.